Americium
[Rn] 5f⁷ 7s² · 2 · 8 · 18 · 32 · 25 · 8 · 2
The one transuranium element millions of people keep at home: americium-241 sits inside ionisation smoke detectors. It was named after the Americas by analogy with europium, directly above it in the table.
3D model
Bohr model: nucleus and electron shells from the real configuration. Valence electrons are highlighted.
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Atomic properties
- Atomic number
- 95
- Atomic mass
- 243 а.е.м. ≈
- Electron configuration
- [Rn] 5f⁷ 7s²
- Electrons per shell
- 2 · 8 · 18 · 32 · 25 · 8 · 2
- Block
- f-block
- Group
- no data
- Period
- 7
- Electronegativity (Pauling)
- 1.13
- Electronegativity (Allen)
- 1.13
- Atomic radius
- 244 pm
- Covalent radius
- 180 pm
- Van der Waals radius
- 244 pm
- Ionisation energy 1
- 578 kJ/mol
- Electron affinity
- 9.9 kJ/mol
- Common oxidation states
- +3
- Oxidation states
- +2, +3, +4, +5, +6, +7
Physical properties
- State at 25 °C
- solid
- Density
- 12 g/cm³
- Melting point
- 1 449 K · 1 175.9 °C
- Boiling point
- 2 880 K · 2 606.9 °C ≈
- Speed of sound
- no data
Thermal properties
- Heat of fusion
- 14.39 kJ/mol
- Heat of vaporisation
- 238.5 kJ/mol
- Specific heat capacity
- 0.11 J/(g·K)
- Thermal conductivity
- 10 W/(m·K)
Mechanical properties
- Young's modulus
- no data
- Shear modulus
- no data
- Bulk modulus
- no data
- Poisson ratio
- no data
- Mohs hardness
- no data
- Brinell hardness
- no data
Electrical and magnetic properties
- Electrical resistivity
- 690 nΩ·m
- Magnetic ordering
- paramagnetic
- Curie point
- no data
- Néel point
- no data
- Superconducting point
- 0.6 K
Crystal structure
- Crystal structure
- hexagonal close-packed (hcp)
- Lattice constants
- a = 346.8 pm · c = 1 124 pm
Abundance
- In the crust
- 0 mg/kg
- In the ocean
- 0 mg/L
- In the universe
- 0 mg/kg
- In the human body
- 0 mg/kg
Isotopes
| Isotope | Abundance | Half-life | Decay mode |
|---|---|---|---|
| 241Am | — | 432.23 yr | α |
| 243Am | — | 7 370.8 yr | α |
Americium-241, half-life 432 years, is the smoke detector isotope. Americium-243, half-life 7370 years, is longer-lived and accumulates in waste.
Discovery
- Year of discovery
- 1944
- Discovered by
- Glenn Seaborg and colleagues
- Where
- Chicago, USA
- Origin of the name
- after the Americas, by analogy with europium directly above it
History
Seaborg and colleagues made americium in late 1944 in Chicago by irradiating plutonium with neutrons. The discovery was first announced on a children's radio quiz show in November 1945, the day before the planned scientific conference.
Where it occurs
Effectively absent from nature.
How it is produced
In reactors, by repeated neutron capture on plutonium. Americium accumulates in spent fuel and in ageing plutonium stocks.
Role in living things
No biological role. It deposits in bone like the other actinides.
Safety
Radiotoxic, but a smoke detector holds under a microgram in a sealed capsule, and the dose from it is negligible against natural background.
Uses
- Ionisation smoke detectors: americium ionises the air and smoke disturbs the current
- Industrial density and thickness gauges
- Neutron sources paired with beryllium for geophysical logging
- A candidate for space power sources, replacing scarce plutonium-238
Curiosities
- A typical smoke detector holds about 0.3 micrograms of americium-241.
- Americium was named by analogy with europium, sitting directly above it — the naming logic was pointedly symmetrical.
- Americium-241 grows out of plutonium-241 on its own, so old plutonium stocks spoil over time.
- Americium and curium were first announced on a children's radio programme, not in a journal.